Phylum: Porifera (sponges)

Life > Eukaryotes > Opisthokonta > Metazoa (animals)

Introduction

The Porifera (sponges) differ from other animals in that they do not exhibit true tissue development but rather have a cellular level of organisation. They are simple in body form, having a sac-like body which is perforated by many pores, yet complex at the cellular level having several types of cells. The outer layer of the cells wall contains flattened epidermal cells, some of which have contractile fibers; the middle layer is a semifluid matrix with wandering amoebocytes; and the inner layer is composed of flagellated cells called collar cells (or choanocytes) that look like protozoans. One could thus argue that sponges are composed of colonies of protozoans living in a symbiotic relationship with each other.

All sponges are sessile, suspension feeding animals with radial or no symmetry. Only their sperm and larvae are motile and once a larva settles it attaches itself to a substrate and develops into an adult. Sponges are filter feeders and are the only animals in which digestion occurs within cells. They may be as tiny as a finger nail to as large as a person. Even though many evolutionary biologists believe them to be an evolutionary dead end, sponges are one of nature’s success stories.

There are over 5000 species of sponges known globally and are generally classified according to their skeletal structures. Their body shape is maintained by possessing spicules, made of calcium carbonate or silicate, or collagenous fibers known as spongin. The class Calcarea possesses calcium carbonate spicules and members thereof are exclusively marine. The class Hexactinellida or glass sponges, primarily found in deep marine habitats, possesses spicules composed of silica. The class Demospongiae, found in marine, brackish and fresh water environments, possesses both spicules and spongin. At present 314 Poriferan species are known from South Africa. This species diversity compares well with most regions that have well documented sponge faunas but is unexceptional when compared to Australia (2426 species), although this later comparison cannot strictly be made due to the vast differences in sampling effort, area and habitat diversity.

Ecology

Sponges are largely predated upon by nudibranchs. In turn they filter out microscopic food particles from the water (taken in through pores in their body walls) that gets pumped out via larger exhalent openings called oscula. As a group they are generally protected against predators by their spicule skeletons, while others slime up or may produce toxins. In fact, some nudibranchs, which feed on certain sponges, have the ability to deter predators because they assimilate these sponge toxins into their own flesh after feeding on them.
 

Sponges are able to form unusual interactions and associations with other organisms. They may bore into the shells of bivalves, gastropods, and the colonial skeletons of corals by slowly etching away chips of calcareous material. Larvae may settle on mollusc shells that are being used by hermit crabs (eg., sponge living on the hermit crab ) and then develop into adult sponges by engulfing the shell completely. Crustaceans like the cryptic sponge crab, Cryptodromiopsis spongiosa, may cut off pieces of unpalatable sponge and hold it over its body with its modified fifth pair of legs. Sponge/crab associations result in sponges that are no longer sessile. These exclusive, species specific, associations can accurately be used by field observers to identify certain species in situ (in the field).

 Porifera and Humans

The original bath sponge was derived from specimens of sponge that have skeletons composed entirely of spongin fibers. However, most modern sponges are made of synthetic substances, but in certain areas of the world sponges are still harvested for this industry. More recently sponges, along with many other organisms, have been discovered to be sources of biomedical compounds. Spongistatin 1 is one such compound which has exhibited antimitotic properties, stunting the growth of cancerous tumors. Sponges are also valuable indicators of marine environmental health.
 

Classification of southern African sponges

Class Calcarea
Class Hexactinellida
Class Demospongiae

Links

• Introduction to Porifera. An excellent illustrated account of the basic morphology, classification, life history and ecology of sponges.

Further reading

• Branch, G. and Branch, M. 1981. The Living Shores of Southern Africa. C. Struik Publishers, Cape Town.

Text by Wayne K. Florence